1. Field of the Invention
This invention relates to a method of making silylphosphates, to the silylphosphates, and their use in stabilizing alkali metals and alkaline earth metals in polyorganosiloxanes.
2. Background Information
Polydiorganosiloxanes are used in many products, such as various kinds of silicone rubbers and fluids. Many of the products require property stability under high temperature exposure to function properly in their intended utility. Because the polydiorganosiloxanes are most often made by a polymerization process involving strong base equilibration of linear polydiorganosiloxane hydrolyzates or cyclic polydiorganosiloxanes and because this equilibration proceeds via a silicon-oxygen-silicon bond breakage and reformation, the basic polymerization catalyst used must be rendered ineffective if it remains in the final product. The amount of the basic compound is very small and difficult to remove at reasonable cost, so techniques have been developed to reduce the catalyst's harmful effects. The methods of neutralizing the catalyst's activity have varied effectiveness and each method seems to have one or more disadvantages.
Polydiorganosiloxane can be prepared by the well known process of converting low molecular weight linear polydiorganosiloxanes and cyclic polydiorganosiloxanes by heating above 100.degree. C. in the presence of potassium hydroxide or potassium silanolate. Other known alkali metal catalysts for this kind of polymerization, are sodium hydroxide, cesium hydroxide, lithium hydroxide, and their corresponding silanolates or siloxanates. In the case of the cyclic polydiorganosiloxane polymerization, a ring opening reaction takes place with the formation of linear polymers. Most often, such as in the case of polydimethylsiloxane the resulting product of the equilibration reaction is about 85% linear polymer and 15% cyclic polydimethylsiloxanes. The presence of the cyclic siloxanes in products is undesirable because they are of low molecular weight and have a sufficiently high vapor pressure to cause problems during use, such as problems in closed or semiclosed conditions where electrical or electronic equipment is in close proximity with silicone rubber and therefore, these cyclic siloxanes should be removed. The most convenient method of removing these cyclic siloxanes is by heating under reduced pressure, however, if the basic catalyst's activity is not hindered, the distillation process will continuously generate cyclic siloxanes; as they are removed from the linear polydiorganosiloxane product more will be formed because of the reaction's potential to go to equilibrium. Therefore, it is important even in the preparation of the linear polydiorganosiloxanes to stabilize the basic catalyst. Various methods of stabilizing this basic catalyst have been used in the past. Terms, such as neutralizing the catalyst or killing the catalyst have been used in the art with various meanings. The inventors in this application use the term stabilizing the catalyst or stabilization of the catalyst to mean reducing the deleterious activity of metal ions resulting from polymerization reactions, thereby making them ineffective, for the most part, to cause Si-O-Si bond rearrangement and cyclic formation.
One method of neutralizing the basic catalyst is the use of various types of acids. One difficulty with strong acids such as hydrochloric acid or sulfuric acid is that the amount of the acid used must be very carefully controlled because either excess base or excess acid will be detrimental to the final linear polydiorganosiloxanes stability. Both acids and bases are known equilibration catalysts, thus both produce similar results if left in the product. It is known that excess acid will cause degradation of the product similar to the degradation resulting from base such as alkali metal hydroxides. It is difficult to get the base completely neutral using a strong acid and would be very expensive and time consuming. Weak acids have also been used, such as acetic acid, but these acids have a similar problem.
Phosphoric acid, because it is a buffering kind of acid, has the ability to overcome the strong acid problem of neutralizing the basic catalysts used in the preparation of linear polydiorganosiloxane through an equilibration reaction. However, phosphoric acid is not soluble in the linear polydiorganosiloxane or in the cyclic polydiorganosiloxanes and to be an effective catalyst stabilizer it needs to be soluble so that it can get to the alkali metal ions which are often located, when equilibrium is reached, on the terminal silicon atoms of the polydiorganosiloxane product as Si-O-M where M is an alkali metal atom. Solvents are not useful because solvents for the phosphoric acid are not solvents for the siloxanes and solvents for the siloxanes are not solvents for the phosphoric acid. To overcome the difficulty with the insolubility of the phosphoric acid, Razzano et al in U.S. Pat. No. 4,177,200, issued Dec. 4, 1979, found a soluble form of phosphoric acid which could be used to neutralize siloxane mixtures containing alkali metal hydroxides. Razzano et al found that the known silyl phosphates made by reacting phosphoric acid and octylmethylcyclictetrasiloxane and a small amount of hexamethyldisiloxane could be used to neutralize alkali metal hydroxide in siloxanes. However, Razzano et al reported two difficulties with this silyl phosphate. The viscosity of the silyl phosphate was too high, greater than 500 centipoise at 25.degree. C. and this made it difficult to blend with the siloxane equilibration reaction mixture. The other difficulty reported was that the phosphoric acid content of the silyl phosphate could only achieve a maximum of 10 to 15% by weight.
Razzano et al describe a silylphosphate made by reacting a siloxane selected from the class of siloxanes of the formula (R.sub.3 Si).sub.2 O and siloxanes of the formula R.sub.3 Si(R.sub.2 SiO).sub.x OSiR.sub.3 with phosphorous oxyhalogens POCl.sub.3 or POBr.sub.3 where R is a hydrocarbyl radical free of aliphatic unsaturation and x varies from 1 to 20. Razzano et al also describe a less preferred method for preparing silylphosphates by reacting phosphoric acid with linear siloxanes at temperatures above 150.degree. C. The advantage given for using phosphoric acid in this case is that less of the linear siloxanes are used up in the formation of the silylphosphates. According to Razzano et al, the reaction of phosphoric acid with the siloxanes is difficult and does not take place readily unless temperatures 150.degree. C. to 200.degree. C. are reached. Razzano et al, in a solvent, reacts 1 mole of phosphoric acid with 1.5 moles or more of the siloxane, preferably from 1.5 to 6 moles of the linear siloxanes per mole of phosphoric acid. The by-produced water is distilled off until the reaction is completed taking 1 to 7 hours. The silylphosphate produced are (R.sub.3 SiO).sub.3 P.dbd.O and {R.sub.3 SiO(R.sub.2 SiO).sub.x }.sub.3 P.dbd.O where R and x are defined above. Razzano et al report that because the reaction is carried out with more difficulty and may not proceed to completion there may be some amounts of monosilyl and disilyl substituted phosphate reaction products and the phosphoric acid will be left with one or two hydroxyl groups. The monosilyl and disilyl substituted reaction products may constitute as much as 10% by weight, preferably not more than 5% by weight of the total reaction mixture. The silyl phosphate reaction product consisting mostly of trisilyl substituted phosphates is used to neutralize the equilibration siloxane reaction mixtures having alkali metal hydroxide.
An improved method for preparing silylphosphates from phosphoric acid and linear low molecular weight polysiloxane is described by Petersen in U.S. Pat. No. 4,125,551, issued Nov. 14, 1978. The method taught by Petersen comprises reacting 1 to 30 parts by weight of phosphoric acid with 100 parts by weight of polysiloxane of the formula R(R.sub.2 SiO).sub.w SiR.sub.3 where R is a monovalent hydrocarbon radical and w is from 1 to 100 in the presence of 1.2 to 180% by weight of the total composition of a silyl phosphate catalyst in which the phosphoric acid equivalent in the reaction mixture is from 0.36 to 1.80%.
Petersen teaches that the reaction is carried out by placing 5 to 25% of the total phosphoric acid in contact with the polysiloxane and the silyl phosphate catalyst, the mixture is heated and the remaining phosphoric acid is added. The reaction began at 150.degree. C. in most cases and varied upwardly during the reaction period until the final temperature of 175.degree. C. to 196.degree. C. was reached. Petersen found that in all cases where no silyl phosphate catalyst was used in the reaction mixture, the reaction did not initiate for a substantial period of time and then the reaction was violent. Petersen describes the product of the method, as a polymer not having a single composition, but a statistical distribution of a variety of structures and molecular weights about a center point.
The process described by Razzano et al which uses POCl.sub.3 or POBr.sub.3 to make silylphosphate makes trisilyl phosphates and has the disadvantage of by-producing large amounts of triorganochlorosilane or triorganobromosilane. The method described by Razzano et al which combines phosphoric acid and siloxane to make silylphosphate is violent as described by Petersen who describes the use of a silylphosphate catalyst to make silylphosphate from phosphoric acid and siloxane.
Czechoslovakian Patent No. 173,332, published May 28, 1976, to Dvorak et al teach that making tris(trimethylsilyl)phosphate in high yields from phosphoric acid and hexamethyldisiloxane requires high pressure and high temperature. For example, the reaction is carried out at a pressure of 1 to 10 atmospheres at a temperature of 200.degree. C. for three hours.